1. Field of the Invention
The invention relates generally to interface circuits for industrial automation systems and, more specifically, to interface circuits for power control and power management in applications such as intelligent power supplies and systems requiring local power device sense and control capability, in addition to the ability to transmit both power and system control information across a high voltage isolation barrier.
2. Description of the Relevant Art
Reduced to bare essentials, an industrial process may be regarded as having a number of sensors and loads that correspond to input and output variables for a process control computer system. The sensors provide input values representative of the state of the process at a given time; the loads respond to output values, and thereby control various aspects of the process. Typical sensors include relay contacts, proximity switches, and pressure switches. Typical loads include contractor coils of starters for large motors, solenoid valves, relays, lamps, and small motors. A process may have several hundred to several thousand input sensors and loads that must be serviced at very frequent intervals.
A typical computer system for automating an industrial process contains a number of general and special purpose computers. The system monitors input variables from the process, performs suitable logical manipulations on the inputs, and updates output variables for the process. The computer system is usually organized hierarchically. A host processor, typically a minicomputer or a mainframe, communicates with a number of programmable controllers, each of which communicates with a number of power control subsystems through a number of local processors. A programmable controller is a processor designed specifically to perform logical manipulations on a large number of binary inputs on a cyclical basis. The local processors have as their primary function the efficient transfer of data between the power control subsystems and the working memories of the programmable controllers. The power control subsystems provide the interface between the local processors and the various sensors and loads.
A factory automation system, such as described above, must be provided with suitable electrical isolation between the local processor side and the high voltages on the load side of the power control subsystems. The possibility of a short circuit between a motor running from any line voltage (AC or DC, single phase or more) and the programmable controller or local processor directing it or the host processor is horrible to contemplate, particularly in terms of economic cost and danger to human life.
Optical isolators are almost always used to provide an isolation barrier able to withstand voltages on the order of 2500 volts AC. Optical isolators have their limitations, however, since they are unidirectional in character, and, if bidirectional communication through the isolation media is desired, two couplers per channel must be provided. Furthermore, optical isolators are unable to deliver power across the barrier. Since optimum flexibility in sensing and controlling demands that a barrier be provided between each local processor and the load side of each of its power control subsystems, as well as between each of the power control subsystems, each subsystem therefore requires a separate isolated power supply. This adds greatly to cost and complexity of the overall system. Finally, the performance of optical isolators degrades with use due to physical shifting of components, so frequent repair and/or replacement of the isolation circuitry is required.